In RTW you had many cities which mostly didn´t come in contact with the enemy. In BI the distances are great so my games shure have changed from RTW-build-as-many-armies-as-you-can-afford to BI-only-offensive-armies style.
Ex: Holding the lands of WRE in RTW meant one stack in Iberia, Britain, France, Italy, Balkan, Africa etc etc. That is atleast 5 armies.
In BI I tend only to build armies that fill a certain purpose offensively, like defending your northern bridges against the hordes as ERE or taking your objective cities etc. My main defence nowdays are my cities and their heavy fortifications. I use 2-4 really good infantry and 4-6 archers in every city I hold (usually) and I haven´t lost a city yet.